This invention relates in general to vehicle disc brake assemblies and in particular to an improved structure for an electric parking brake actuator for use in such a disc brake assembly.
Most vehicles are equipped with a brake system for slowing or stopping movement of the vehicle in a controlled manner. A typical brake system for an automobile or light truck includes a disc brake assembly for each of the front wheels and either a drum brake assembly or a disc brake assembly for each of the rear wheels. The brake assemblies are actuated by hydraulic or pneumatic pressure generated when an operator of the vehicle depresses a brake pedal. The structures of these drum brake assemblies and disc brake assemblies, as well as the actuators therefore, are well known in the art.
A typical disc brake assembly includes a brake rotor which is secured to a wheel of the vehicle for rotation therewith. The disc brake assembly further includes a caliper assembly that is slidably supported on pins secured to an anchor bracket. The anchor bracket is secured to a non-rotatable component of the vehicle, such as the axle flange or steering knuckle, which is connected to the vehicle frame. The caliper assembly includes a pair of brake lining assemblies which are disposed on opposite sides of the brake rotor. The brake lining assemblies are connected to one or more hydraulically or pneumatically actuated pistons for movement between a non-braking position and a braking position, wherein they are moved into frictional engagement with the opposed braking surfaces of the brake rotor. For example, when an operator of the vehicle depresses the brake pedal, the piston urges the brake lining assemblies from the non-braking position to the braking position so as to frictionally engage the opposed braking surfaces of the brake rotor and thereby slow or stop rotation of the associated wheel of the vehicle.
The disc brake assembly may also include a single electromechanical actuator that is used to realize a parking brake function of the disc brake assembly. The electromechanical actuator may drive a rotationally restrained nut threaded onto a motor driven spindle. As the spindle is rotationally driven, the nut axially translates to move the piston and urge the brake lining assemblies between the braking and non-braking positions. The piston may be hollow with the nut and spindle located inside the piston.
The disc brake assembly may comprise multiple pistons, each of which has an individual nut and spindle. To realize the parking brake function with multiple pistons, the electromechanical actuator may drive only a single nut and spindle of a single piston. However, when only a single nut and spindle is driven, the single actuator must be large in size to realize the parking brake function. Alternatively, the single actuator, through a gearing, may drive multiple nuts and spindles located in multiple pistons. However, the gearing fixes the displacement of the nuts on the spindles such that the cylinders all displace the same fixed amount. The fixed displacements of the cylinders combine with non-uniform pad wear—e.g., taper wear—of the brake lining assembly to result in the cylinders producing unequal clamping force. Therefore, it would be desirable to have a disc brake assembly having independently controlled actuators for the parking brake function.